Basicity elimination

Previous syntheses of terminal alkynes from aldehydes employed Wittig methodology with phosphonium ylides and phosphonates. 6 7 The DuPont procedure circumvents the use of phosphorus compounds by using lithiated dichloromethane as the source of the terminal carbon. The intermediate lithioalkyne 4 can be quenched with water to provide the terminal alkyne or with various electrophiles, as in the present case, to yield propargylic alcohols, alkynylsilanes, or internal alkynes. Enantioenriched terminal alkynylcarbinols can also be prepared from allylic alcohols by Sharpless epoxidation and subsequent basic elimination of the derived chloro- or bromomethyl epoxide (eq 5). A related method entails Sharpless asymmetric dihydroxylation of an allylic chloride and base treatment of the acetonide derivative.8 In these approaches the product and starting material contain the same number of carbons. 

A 3-Phosphorin derivatives of benzo annelated rings were intensively studied by Bickel-haupt and coworkers. One of his preferred methods consists in the introduction of the final P=C double bond from appropriate ring systems by basic elimination of HC1 with DBU or other bases. Many benzo-A 3- phosphorins have been reported. An example is the synthesis of 2-phosphanaphthalene (equation 26) (75T1097). Some other examples are shown in (32)-(35). 

Basic elimination of a good leaving group is not utilized in construction of the double bond... 

Walraevens, R., Tromhet, P., Devos, A. (1974) Basic elimination of hydrogen chloride from chlorinated ethanes. Int. J. Chem. Kinet. 6, 777-786. 

Because carboxylate salts are only weakly basic, elimination is not a problem when the leaving group is attached to a primary or secondary carbon. Several examples are provided in the following equations ... 

The reaction of a-terf-butyldiphenylsilyl carbonyl compounds 4.73 with organometallics occurred with a high diastereoselectivity to give erythro- P -hydroxysilanes 4.74, which under acidic and basic elimination conditions gave E- and Z-alkenes, respectively (Scheme 4.44). ... 

Mixture (depends on basic.) Elimin. by fast E2orEl... 

Overall, we start with basic elimination reactions that will break our starting hemiacetal. We know at the end of the mechanism will be an addition reaction that forms the product hemiketal. What happens in between is uncertain at this point, but we have a rough idea of where to start and where to end on our problem space (Fig. 10.10). 

Thus the ether 3 rearranges to 4 with high (E)- and o/iri-selectivity to give 4. This product is reduced by sodium bis(2-methoxyethoxy)aluminum hydride to the (1E,5E)-diene 5. Acidic elimination followed by protiodesilylation gives sarohornene B (6) in high selectivity. In contrast, basic elimination and protiodesilylation provides sarohornene C (7) with high selectivity. 

Because the leaving group is attached to a secondary carbon and methoxide is strongly basic, elimination predominates. However, note that the weakly basic azide ion reacts with the secondary p-toluenesulfonate shown by substitution (See Problem 8.43). 

This reaction is a Favorskii trmisposition followed by a basic elimination of HBr. It begins with the formation of an anion on a to the ketone, which displaces one of the bromine atom, giving a cyclopropanone. 

The basic elimination of HBr leads to an alkene, which is stabilized by conjugation with two esters. 

This reasoning outlines a basic elimination procedure for the construction of candidate ARs. Each extreme point C,- X, on the current convex polytope Pj., is checked for feasibility via backward CSTR solutions and PFR trajectories. If either the backward CSTR or PFR trajectory from C,- intersects the current polytope boundary, then it is retained. Otherwise, C, is removed from the current set of extreme points... 

Nelson and co-workers reported cinchona alkaloid-catalyzed [4-1-2] cycloaddition of ketenes and N-thioacyl imine, affording the 4,5-cw-disubstituted l,3-thiazin-6-one derivatives 146 with high enantioselectivities (>95% ee) and diastereoselectivities (>95 5 cis. trans). Scheme 3.47 [63], Ketene, in situ generated from acyl halide 143 and base, followed by addition to imine which was generated in situ via basic elimination of a-amido sulfone 144, providing the ketene-imine addition pathway toward the cycloadducts. 

This reactivity indicates that, while appropriate for the introduction of moderate strain, smaller CPPAs are unobtainable by basic elimination [14]. 

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